1. Field of the Invention
The present invention relates to a cutting insert releasably attached to a tool body of a cutting tool such as a face milling cutter, and in particular to an insert which is secured to the tool body such that one of the opposite faces facing in the direction of the insert thickness is directed radially outwardly with respect to the tool body. The present invention also relates to an insert cutter employing the above cutting insert.
2. Prior Art
In cutting tools such as a face milling cutter, a so-called vertical insertion type is hitherto known in which a polygonal plate-like insert is secured to a tool body such that one of the opposite faces directed in the direction of the insert thickness is directed radially outwardly with respect to the tool body. FIGS. 16 to 18 depict a prior art cutting insert for use in the cutting tools of this kind. This insert is disclosed in Japanese Patent Application, Laid-Open No. 53-101792, and comprises an insert body 1 formed by molding a hard material such as cemented carbide into a rhombic plate-like shape. The insert body 1 is defined by opposite front and rear faces 2 and 2 directed in the direction of the insert thickness and four peripheral faces 3 lying between the opposite faces 2 and 2 so as to extend perpendicular to the faces 2 and 2. Among marginal ridges of the two opposite faces 2 and 2, four marginal, edge which are alternately disposed at the opposite faces 2 and 2, in a staggered relation to one another, define main cutting edges 4, respectively.
Furthermore, the corner portion of the peripheral face 3, into which a marginal edge 5, defined by the two adjacent peripheral faces 3 and 3 of the insert body 1 and located at a respective acute corner portion C1 of the insert body 1, and a marginal edge of the front or rear face 2, defining no main cutting edge, merge, defines an inclined surface 6 having a width increasing toward the aforesaid marginal edge 5. The marginal edge of the inclined surface 6 which serves as a portion of the marginal edge 5 defines an auxiliary cutting edge 7 which intersects the main cutting edge 4 so as to form an obtuse angle therebetween. In addition, a rake surface 8 extending along the main and auxiliary cutting edges 4 and 7, is formed on the peripheral face 3 which has as its marginal edges both the marginal edge 5 defining the main cutting edge 4 and the marginal edge defining the auxiliary cutting edge 7.
Furthermore, in FIGS. 16 to 18, the numeral 9 denotes a nose portion formed at a portion into which the main cutting edge 4 and the auxiliary cutting edge 7 merge, and the numeral 10 denotes a chamfered portion formed on the marginal edge which does not serve as the main cutting edge 4. In addition, the numeral 11 denotes a mounting aperture for releasably mounting the insert body 11 to a cutting tool such as a face milling cutter.
In a face milling cutter as shown for example in FIG. 19, the prior art cutting insert constructed as described above is secured to an outer periphery of a tool body 12 by means of a clamp screw 13, in such a manner that one of the opposite faces 2 and 2 is directed in a radially outward direction with respect to the tool body 12, and that one of the main cutting edges 4 formed on the marginal edges of one face 2 is indexed so as to protrude from the outer peripheral surface of the tool body while the auxiliary cutting edge 7 extending continuously from the indexed main cutting edge 4 is indexed so as to protrude from the forward end face of the tool body. As shown in FIG. 19, the rake surface 8 extending along the indexed main and auxiliary cutting edges 4 and 7 is directed in the direction of rotation T of the tool body 12, and in particular with regard to the indexed main cutting edge 4, the rake surface is a negative rake surface which intersects the front face 2 so as to form a 90.degree. angle therebetween. Furthermore, the one face 2 extending from the indexed main cutting edge 4 serves as a flank surface for the main cutting edge 4, whereas the inclined surface 6 formed on the peripheral face 3, which is directed in a forward direction with respect to the tool body 12, serves as a flank surface for the indexed auxiliary cutting edge 7.
Moreover, with the above construction, the aforesaid prior art insert may be used four times by rotating it 180.degree. or by turning it over causing unused cutting edges to be indexed.
In the cutting insert as described above, it is well known that in order to improve cutting performance, cutting load to be exerted on the main cutting edge 4 must be reduced, and thus accordingly the axial and radial rake angles for the indexed main cutting edge 4 should be increased in their positive directions.
However, in the prior art insert as described above, the peripheral face 3, on which the rake surface for the indexed main cutting edge 4 is formed, is formed as a single planar surface, perpendicular to the face 2 over the entire length of the main cutting edge 4. As a result, in order to increase the axial rake angle for the main cutting edge 4 while ensuring a sufficient relief angle for the auxiliary cutting edge 7, the apex angle at the acute corner portion C1 of the insert body 1 (i.e., the angle defined between the peripheral faces 3 and 3 intersecting each other at the acute corner portion C1) must be made small. However, if the insert is formed in a more oblate rhombic shape, the included angle of the auxiliary cutting edge 7 at the acute corner portion C1 becomes unduly small. Therefore, the strength of the cutting edge is decreased and the cutting edge becomes susceptible to chipping.
In addition, a positive cutting insert in which a rake surface extending along the main cutting edge is formed so as to intersect one of opposite faces at an acute angle formed therebetween in order to increase the radial rake angle for the main cutting edge, is also known. However, in such a positive cutting insert, since the included angle of the main cutting edge is small, the possibility of chipping due to the reduction of the strength at the tip end of the cutting edge cannot be avoided.
In particular, when a cutting insert is employed for the face milling cutter as described above, the cutting edge which protrudes axially forwardly with respect to the tool body, will be subjected to the largest load during the cutting operation, and therefore the problems regarding the chipping of the cutting edge are very serious.